1. Field of the Invention
The present invention relates to an apparatus and method for determining time-resolved voltage at a node in an integrated circuit using optical observation.
2. Description of the Related Art
It is known in the prior art that various mechanisms in semiconductor devices can cause light emission. Detection of such light emission has been used to investigate semiconductor devices. For example, avalanche breakdown in insulators causes light emission, and detection of such light emission can point to the locations of failure in the device. Similar detection can be used to characterize electrostatic discharge in the device. In electrically stimulated (active) transistors, accelerated carriers (electrons & holes), i.e., hot-carriers, emit light when the device draws current. Various emission microscopes have been used for detecting locations on the device where the electrical current drawn exceeds the expected levels and therefore could lead to locating failures in semiconductor devices. Examples of emission microscopes may be found in U.S. Pat. Nos. 4,680,635; 4,811,090; and 5,475,316.
For transistors, such as those in complementary metal oxide semiconductor (CMOS) devices, the current “pulse” coincides (in-time and characteristics) directly with the voltage transition responsible for the change in the state (logic) of the device. Therefore, resolving in time the hot-electron emissions from electrically active semiconductor transistor devices indicates the behavior and response of the device to electrical currents and the temporal relations of the current pulses with respect to each other. These temporal characteristics, along with the detection of the transition (pulse) itself, are of critical importance in the design and debug of integrated circuit (IC) devices. Related works on the subject have been published and represented by the following papers:
All-Solid-State Microscope-Based System for Picosecond Time-Resolved Photoluminescence Measurements on II–VI semiconductors, G. S. Buller et al., Rev. Sci. Instrum. pp. 2994, 63, (5), (1992);
Time-Resolved Photoluminescence Measurements in InGaAs/InP Multiple-Quantum-Well Structures at 1.3-m Wavelengths by Use of Germanium Single-Photon Avalanche Photodiodes, G. S. Buller et al., Applied Optics, Vol 35 No. 6, (1996);
Analysis of Product Hot Electron Problems by Gated Emission Microscope, Khurana et al., IEEE/IRPS (1986);
Ultrafast Microchannel Plate Photomultiplier, H. Kume et al., Appl. Optics, Vol 27, No. 6, 15 (1988);
Two-Dimensional Time-Resolved Imaging with 100-ps Resolution Using a Resistive Anode Photomultiplier Tube, S. Charboneau, et al., Rev. Sci. Instrum. 63 (11), (1992); and
Dynamic Internal Testing of CMOS Circuits Using Hot Luminescence, J. A. Kash and J. C. Tsang, IEEE Electron Device Letters, vol. 18, pp. 330–332, 1997.
Notably, Khurana et al., demonstrated that photoluminescence hot-carrier emission coincides in time and characteristics with the current pulse, i.e., the voltage switching of a transistor, thereby teaching that, in addition to failure analysis (location of “hot-spots” where the device may be drawing current in excess of its design), the phenomenon can also be used for obtaining circuit timing information (switching) and therefore used for IC device debug and circuit design. See, also, U.S. Pat. No. 5,940,545 to Kash et al., disclosing a system for such an investigation. For more information about a time-resolved photon emission system the reader is directed to U.S. Pat. No. 6,621,275, commonly assigned to the current assignee and incorporated herein by reference in its entirety. Such a system is commercially available under the trademark EmiScope® from assignee, Optonics Inc., a Credence Company, of Mountain View, Calif.
FIG. 1 is a block diagram depicting an arrangement of a conventional time-resolved emission system. A device under test (DUT) 110 is being stimulated by stimulus 120, e.g., a conventional automated testing equipment (ATE 205 in FIG. 2). The ATE also sends a start signal to the time-to-digital converter 180, so that it is synchronized therewith. When the DUT emits light in response to the stimulus 120, the light is detected by detector 150, which then sends a signal to the time-to-digital converter 180, so that the timing of the emission can be determined.
FIG. 2 depicts the general elements of the system, as it is coupled to a commercially available ATE 205. The ATE 205 generally comprises a controller, such as a pre-programmed computer 281, and a test head 224, which comprises an adapter 225 used to deliver signals programmed by the controller 281 to the DUT (not shown) in a manner well known in the art. Specifically, the ATE is used to generate signals that stimulate the DUT to perform various tasks, as designed by the chip designer to check and/or debug the chip. The various signals generated by the controller test head 224 are programmed by the controller 281 and are delivered by the test head 224 to the DUT via the adapter 225. The adapter 225 may include a space transformer, a DUT load board and a DUT socket, in a manner well known in the art.
In the embodiment depicted in FIG. 2, the ATE test head 224 is placed over opening 285 on the top of a vibration isolated bench 215. Chamber 200 houses the main components of the diagnostic system, and is situated below, so that once the ATE head 224 is connected to the system, no external light can reach the elements inside chamber 200. The diagnostic system is controlled by controller 280, such as a pre-programmed general-purpose computer, which also communicates with the ATE controller 281.
FIG. 3 depicts an illustrative circuit having two complementary switching elements. As the voltage, Vg, on the gate changes, light, hυ, is emitted. The voltage rise/fall to be investigated is marked at V(t), while the voltage of the drain of the switching element is marked as Vd.
FIG. 4 is a graph showing at the top a plot of voltage versus time, V(t), and on the bottom a plot of light emission, L(t), by the switching element. From FIG. 3, one can appreciate that V(t) directly correlates to Vd. As depicted, as the element switches, the voltage across it, Vd, drops and, at some point during the voltage drop, t1, the element emits a pulse of light that can be detected by the system of FIGS. 1 and 2. Similarly, when the element reverse-switches, the voltage, Vd, increases and at a certain time during the voltage rise, t2, a pulse of light is emitted. The light emission L(t) is generally given by:L(t)≈Vx(t)I(t) Exp[−Vc/Vx(t)];Vx(t)≈Vd(t)−Vd,sat(t);Vd,sat≈Vg(t)−Vthwhere Vc is a constant specific to the device under investigation (a function of the device structure, material composition, manufacturing process etc.), Vd is the voltage at the drain, Vd,sat is the saturation voltage at the drain, Vg(t) is the voltage at the gate, and Vth is a threshold voltage.
As can be appreciated, the light pulse is narrower in time than the voltage drop/rise, and provides no information about the “behavior” or “history” of the voltage change. In particular, the emission is highly nonlinear in the voltage Vd and is not generally useful for obtaining quantitative voltage information. Obtaining such a linear indicator of the voltage at all times is of great interest to chip designers. Additionally, as is well known, since the current drawn by the device of interest is very small, the emission of the switching device is very faint, requiring single photon detection technology and repeated measurement for obtaining accurate timing of the emission.